1. Field of the Invention
This invention relates to fluid additive supply systems for fire fighting mechanisms, and in particular to systems for adding foam concentrate into a waterline (handline that supplies a nozzle used for fire fighting operations).
2. Description of Related Art
Fire fighting mechanisms typically comprise of a source of water, the primary fire fighting fluid, connected to a water pump that supplies water under suitable pressure through a conduit (fire hose) to a monitor or hand held nozzle. It is often desirable also to have a portable mechanism that can supply an additive, such as foam concentrate, into the water line. The additive mechanisms may have a metering device that allows the proportioning to be varied from 0 to 6%.
The additive mechanisms may employ the basic design and principle of a venturi device and are typically called eductors or jet pumps by those familiar with this art. Such mechanisms use primary liquid flow to create a pressure drop across an orifice. The additive is drawn into a suction port inlet that is connected to the device in the low pressure zone that is created by this pressure drop.
There are numerous systems on the market for supplying additives to water lines. The majority of these systems are not portable. Many require additive pumps, such as foam concentrate additive pumps, for forced injection or induction. Additive pump systems are more complicated and expensive than eductive systems. By contrast venturi type eductors are economical and reliable. Such eductors are the focus of this invention.
The basic venturi eductors generally fall into one of two categories: by-pass, and non-by-pass eductors. By-pass eductors offer more versatility by allowing the drive water to "by-pass" the orifice or pressure drop area. In this mode, no additive is supplied to the water line. Additionally, since water is not forced through the pressure reducing orifice, more pressure and flow is available to the nozzle for water operations. (See FIG 1B) Pressure drop may be considered negligible in this mode. However, traditionally, by-pass systems are large and heavy and do not easily lend themselves to portable applications. (FIGS. 2A and B).
Non-by-pass eductors are considered to be the most basic of additive devices. They are inexpensive, lightweight and portable (FIG. 3) However, they operate in the "pressure drop" mode. This means that an unrecoverable pressure drop occurs across the orifice device, regardless of whether an additive is being drawn in or not. This permanent pressure loss is typically between 40% to 50% of the working inlet pressure. Such pressure drop comprises a waste of energy if additive is not desired, as is the case in water only applications.